The present invention relates to medical imaging, and in particular to methods of in vivo elastographic imaging.
Elastography is a new imaging modality that reveals the stiffness properties of tissues, for example, axial strain, lateral strain, Poisson's Ratio, Young's Modulus or other common stiffness measurements. The measurements provide a two-dimensional array of data in which array location corresponding to tissue locations in an image plane. The array of data may be mapped to a gray scale to form a picture.
In “quasi-static” elastography, two images of the tissue are obtained. The first image forms a base line of the tissue in an unstressed or relaxed state or under a small pre-compression. The second image is obtained with the tissue under compression. Displacement of the tissue between the two images is used to deduce the stiffness of the tissue. The quasi-static elastogram is analogous to a physician's palpation of tissue in which the physician determines stiffness by pressing the tissue and detecting the amount that the tissue yields under pressure.
In “dynamic” elastography, a low frequency vibration is applied to the tissue and the velocity of the resulting elastic waves is measured, for example, using ultrasonic Doppler detection. Elastography of both types may be conducted using imaging techniques other than ultrasound, including computed tomography (CT) and magnetic resonance imaging (MRI).
While elastography has great promise, in vivo elastography of soft tissue structures such as the liver and other abdominal organs can be difficult because externally applied axial compression may induce nonaxial or lateral slippage of the organ. This lateral motion can obscure true axial tissue compression used to deduce stiffness of the tissue.